Thermoionic converter

ABSTRACT

A thermoelectric converter diode in which the thermal energy required for the heating of the ionisable fluid which fills the space between the emitter and the collector is taken from the collector by conduction and from the emitter by radiation. To this end the ionizable substance is stored in a block having a large surface-to-volume ratio, which block is supported by the collector within the diode.

United States Patent [191 Defranould et al.

[ THERMOIONIC CONVERTER [75] Inventors: Philippe Defranould; Jean Lalaurie;

drvind Shroff, all of Paris, France [73] Assignee: Thomson-CSF, Paris, France [22] Filed: Sept. 8, 1972 21 Appl. N0.: 287,235

Related U.S. Application Data [63] Continuation of Ser. No.-84,261, Oct. 27, 1970,

abandoned.

[52] U.S. Cl. 310/4 [51] Int. Cl. H0lj 45/00 [58] Field of Search 310/4 [56] References Cited UNITED STATES PATENTS 3,454,797 7/1969 Devin et al. 310/4 [4 1 Feb. 19, 1974 3,191,076 6/1965 Huber et al. 310/4 3,426,221 2/1969 Harbaugh 3,452,224 6/1969 Hernquist et al. 310/4 Primary ExaminerD. F. Duggan Attorney, Agent, or Firm-Edwin E. Greigg [57] ABSTRACT A thermoelectric converter diode in which the thermal energy required for the heating of the ionisable fluid which fills the space between the emitter and the collector is taken from the collector by conduction and from the emitter by radiation. To this end the ionizable substance is stored in a block having a large surface-to-volume ratio, which block is supported by the collector within the diode.

2 Claims, 2 Drawing Figures THERMOIONIC CONVERTER This is a Continuation of application Scr. No. 84,261, filed Oct. 27, l970, now abandoned.

The present invention relates to discharge tubes enabling direct conversion of thermal energy to electrical energy.

Generally, this kind of converters take the form of a diode with two electrodes in an internal atmosphere of an ionizable gas or vapour, for'example cesium vapour. When a temperature difference is created between these two electrodes, thchotter, i.e. the emitter electrode, emits electrons which are picked up by the cooler, i.e. the collector, due to'the dual action of the cesium vapour which on the one hand lowers the work function of the electrons of the emitter electrode, and on the other hand, in its ionized condition, neutralises the space charge about this electrode, which would otherwise inhibit electron emission.

The internal pressure is in the order of l to 10 mm of mercury and is determined by heating the separate reservoir containing the cesium to a suitable temperature.

This two-part design with two different operating temperatures of the known converter diodes presents serious drawbacks both in respect of their manufacture and of their operation, as will be described hereinafter.

It is an object of the invention to avoid these drawbacks. v

According to the invention there is provided, in a discharge tube for converting thermal energy to electrical energy, a first and a second electrode, electrically insulated from and having outer surfaces opposite one another, said first electrode being arranged for being raised in operation to a temperature higher than said second electrode, and a storing block of a refractory material having a large surfaceto-volume ratio, capable of accomodating a substance ionizable upon heating, and supported by said second electrode.

For a better understanding of the invention and to show how the same may carried into effect, reference will be made to the drawing accompanying the following description and in which:

FIG. 1 illustrates a converter tube designed in accordance with the prior art,

FIG. 2 illustrates a converter tube in accordance with the present invention.

The converter tube diagrammatically shown in FlG. 1 comprises an emitter 1 (shown in dotted line in the figure), a collector 2 formed by the body of the diode itself and connected by an insulating sleeve to the emitter, and a reservoir 4 for liquid cesium attached to the diode body by a tube 5.

The temperature involved are respectively 2,000K in the case ofthe emitter 1, this being produced by any suitable heat source as is well known in the art, l,0OK for the collector 2, and 600K for the reservoir which is heated by means of a winding 6 connected to :1 voltage source 7. The tube is given a length and a section such that, bearing in mind the material of which it is made, it provides thermal decoupling between the two parts of the system, in order to prevent their temperatures from equalising.

This requirement for thermal decoupling necessitates a structure which is generally rather long, and comprises several separate elements. The communication between the reservoir and the diode proper is, moreover, rather fragile.

As far as operation is concerned, the known structure suffers from significant drawbacks:

The need to supply thermal energy to heat up the ccsium reservoir means that an independent heating device with a precisely controlled temperature is required, and this results in a reduction in the overall efficiency of the conversion operation.

The voltage generated is collected between the points 8 and 9 at the ends of the diode. Because of the substantial length of the tube 5 and because of the substantial resistivity of the refractory metals of which it is made, there is a parasitic ohmic drop, producing a substantial reduction in overall efficiency, in particular when several diodes are connected in series.

Also, as far as manufacture is concerned, the brazing which is required between refractory materials, the pumping, de-gasing and filling with cesium are lengthy and complicated operations.

FIG. 2 illustrates a converter diode in accordance with the invention. It is formed by two coaxial cylinders ,l1 and 12, respectively forming the emitter and the collector and insulated from one another by a ceramic, for example alumina, sleeve 13 which is brazed therebetween through the medium of sleeves l4 and 15 of niobium; a metal cylinder 16 protects the ceramic sleeve 13 against contamination and the high temperature produced by the emitter. At the opposite end, the emitter is equipped with a centering projection 17 which penetrates into. the central orifice of a perforated molybdenum centering disc 18 secured in position by ceramic insulating rings 19 and electrically insulated from the collector by a cylindrical sleeve 20.

The emitter and thecollector are entirely conventional as to their structure and operation made.

A cap 22, containing a threaded hole 23 for fixing the diode, closes off the top end of the collector.

The quantity of cesium required for operation is stored within an annular block of refractory material 21., located between the cap 22 and the collector 12.

Storage is effected at the surface of the material so that a structure with a large developed area is required for this purpose.

In accordance with the invention, it is possible to use a block of pyrolytic graphite which can be produced in the form of thin superimposed layers, creating a kind of lamination with a large surface-to-volume ratio", the cesium forms a chemical compound in the graphite and, when heated, liberates cesium in the vapour state.

In accordance with a variant embodiment, one may use'a metal block made of tungsten or molybdenum powder, produced by sintering and having a large developed surface area; the cesium is stored without any chemical reaction at the surface, in adsorbed form, and is liberated in the vapour state on heating.

In both cases, the temperatures which can be applied to the storing block in order to produce the vapour tension necessary for operation of the converter device are much higher than isthe case when using the conventional liquid cesium reservoir and this enables the storing block to be located inside the converter diode.

In operation, a part of the thermal energy from the emitter and a part of the thermal energy from the collector which reach the annular body 21, by radiation andconduction respectively, raise the latter to the temperature necessary for liberating the cesium vapour at the optimum pressure which is, for example, between 4 and 10 mm of mercury.

The use of a separate heat source is therefore no longer necessary and the short diode structure reduces the ohmic drop between the points at which the generated voltage is collected.

The manufacture of a diode in accordance with the invention has the advantage that it lends itself well to industrial processes and in particular automatic methods of production.

First of all, the assembly 13-14-15 is formed by brazing. The sleeve 13 is for example, of high purity alumina.

The sleeves 14 and 15 are of niobium. The brazing is performed for example by means of a titanium-niobium alloy.

Also a sleeve 13 of metallized alumina can be used, the sleeves 14 and 15 being of niobium and the brazing being performed by means of a high purity coppernickel alloy. These examples are in no way limitative of the types of materials which can be used.

The connection between the ring 13 and the collector on the one hand and the emitter on the other is effected by electron bombardment.

The centering assembly l81920 is then placed in position on the central projection 17 of the emitter. The storing block in the form, for example, of one or more washers of graphite or some other similar adsorbent material, is then placed in'position. A metal getter, of the zirconium-titanium or niobium type for example, can alsobe placed in position at this time in order to maintain the internal pressure at its optimum level.

Each of the elements will previously have been degascd under vacuum at temperature in excess of the opcrating temperature. Cesium is introduced under vacuum through a capillary tube equipped with a metering valve connected to a large-capacity reservoir and, as described above, adhers, with or without chemical reaction, to the surfaces of the block 17.

Finally, the cap is placed on the diode and welded in position under vacuum by electron bombardment.

From the foregoing it will be readily seen that the manufacture of the converters may take place bymeans of automatic rotary machines.

Of course the invention is not limited to the embodiment described and shown which was given solely by way of example.

What is claimed, is:

1. A discharge tube for converting thermal energy to electrical energy and having a compact structure, the tube comprising, in combination: first and second cylindrical electrodes, said electrodes being hollow and coaxially disposed with respect to one another; means for electrically insulating said electrodes from each other, said means for insulating being provided between said electrodes and said first electrode being arranged to be raised in operation to a temperature higher than said second electrode; and a single cesium storing block made of pyrolytic graphite, said second electrode surrounding said first electrode and supporting, at one of its ends, said single cesium storing block made of pyrolytic graphite, said block having an internal structure in the form of thin superimposed layers and said first electrode including at one end a projection extending adjacent said block for heating said block by radiation to thereby liberate cesium in a vapor state.

2. A discharge tube for converting thermal energy to electrical energy and having a compact structure, the tube comprising, in combination: a first elecrode and second electrode in a form of respective, hollow, coaxial cylinders; means for elecrically insulating said coaxial cylinders, said means for insulating being provided between said first electrode and said second electrode; means coupled to said first electrode for raising its temperature higher than that of said second electrode, said second electrode surrounding said first electrode; a single cesium storing block made of pyrolytic graphite, said second electrode supporting, at one of its ends, said single cesium storing block made of pyrolytic graphite, said block having an internal structure in the form of thin superimposed layers, having a large sur face-to-volume ratio, said first electrode including means adjacent said block for radiantly heating said block to liberate cesium and said means for insulating said electrodes including a ceramic sleeve brazed be tween said first and said second electrodes through the medium of sleeves of niobium; and a metal cylinder disposed between said ceramic sleeve and said first electrode for protecting said ceramic sleeve from said first electrode. 

1. A discharge tube for converting thermal energy to electrical energy and having a compact structure, the tube comprising, in combination: first and second cylindrical electrodes, said electrodes being hollow and coaxially disposed with respect to one another; means for electrically insulating said electrodes from each other, said means for insulating being provided between said electrodes and said first electrode being arranged to be raised in operation to a temperature higher than said second electrode; and a single cesium storing block made of pyrolytic graphite, said second electrode surrounding said first electrode and supporting, at one of its ends, said single cesium storing block made of pyrolytic graphite, said block having an internal structure in the form of thin superimposed layers and said first electrode including at one end a projection extending adjacent said block for heating said block by radiation to thereby liberate cesium in a vapor state.
 2. A discharge tube for converting thermal energy to electrical energy and having a compact structure, the tube comprising, in combination: a first elecrode and second electrode in a form of respective, hollow, coaxial cylinders; means for electrically insulating said coaxial cylinders, said means for insulating being provided between said first electrode and said second electrode; means coupled to said first electrode for raising its temperature higher than that of said second electrode, said second electrode surrounding said first electrode; a single cesium storing block made of pyrolytic graphite, said second electrode supporting, at one of its ends, said single cesium storing block made of pyrolytic graphite, said block having an internal structure in the form of thin superimposed layers, having a large surface-to-volume ratio, said first electrode including means adjacent said block for radiantly heating said block to liberate cesium and said means for insulating said electrodes including a ceramic sleeve brazed between said first and said second electrodes through the medium of sleeves of Niobium; and a metal cylinder disposed between said ceramic sleeve and said first electrode for protecting said ceramic sleeve from said first electrode. 